Metal-air cells have been recognized as a desirable means by which to power portable electronic equipment such as personal computers. A power supply with metal-air cells would be preferred by consumers because such cells have a relatively high power output with relatively low weight as compared to other types of electrochemical cells. Metal-air cells utilize oxygen from the ambient air as a reactant in the electrochemical process rather than a heavier material such as a metal or metallic composition.
Metal-air cells use one or more air permeable cathodes separated from a metallic anode by an aqueous electrolyte. During the operation of the cell, such as a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode and reacts with the hydroxide ions, such that water and electrons are released to provide electrical energy.
Recently, metal-air recharging technology has advanced to the point that metal-air cells are rechargeable and are useful for multiple discharge cycles. An electronically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged back to the atmosphere through the air-permeable cathode and hydrogen is vented out of the cell.
Metal-air cells may be arranged in multiple cell battery packs to provide a sufficient amount of power output for devices such as computers. An example of a metal-air power supply is found in commonly owned U.S. Pat. No. 5,354,625 to Bentz, et al., entitled "Metal-Air Power Supply And Air Manager System, And Metal-Air Cell For Use Therein," the disclosure of which is incorporated herein by reference.
Attempts to increase even further the power output of metal-air cells have had mixed results. Increasing the power output of a cell usually involves operating the cell at a higher current drain. Such a higher load, however, can significantly decrease the total energy density of the system and greatly increase the production of heat, both of which are detrimental to efficiency and lifetime of the cell.
It has been suggested that the energy density and heat problems can be overcome in an increased power cell by placing an air cathode on either side of the anode, i.e., a dual air electrode cell. (The present invention is properly described as a "dual air electrode" cell, rather than a "dual cathode" cell, because the function of the cathodes and the anode is reversed during the recharging process.) Such a dual air electrode design would increase the available surface area of the cathode material and should reduce the impedance of the system as a whole.
Known dual air electrode designs, however, suffer from several deficiencies such as how to vent the cell without causing excessive self-discharge and even how to fill the cell efficiently with electrolyte. Other problems that have been encountered include limited capacity retention, electrolyte leakage, and excessive water vapor loss.
Another problem in known designs is "slumping" of the anode, or the escape of zinc, once it is discharged to zinc oxide, from the top side of the anode to the bottom side. Slumping contributes to capacity loss, operating voltage loss, and may cause an imbalance in current distribution between the cathodes. Although known designs have attempted to localize the problem of slumping by using a honey-comb shaped anode, these designs do not eliminate the flow of zinc oxide.
Accordingly, there is a need for an increased power output from a metal-air power supply without compromising the efficiency and lifetime of the cell. Associated with this goal of efficiency and long life is the elimination of slumping in the anode. These goals must be accomplished in a cell that remains light-weight and relatively inexpensive for widespread consumer use in any type of portable electronic device.